1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Background Art
In image forming apparatuses employing electrophotography, the surfaces of image bearing members installed in the apparatus are abraded by frictional sliding contact with a cleaning blade and developing agents and the photosensitive layer of the image bearing member is fatigued by repetitive charging and discharging, which results in deterioration of the image bearing member over time. The image bearing members that produce defective images beyond tolerance because of the deterioration over time are determined to have reached the end of their working life.
In general, image bearing members are replaced before the end of their working life. The time when image bearing members are replaced is typically set as follows: Preliminarily, a test machine having the same configuration as a target machine is subjected to an endurance test and used in a typical environment and under normal use conditions until the image bearing member has come to the end of its working life. In this test, the total number of output images, the total cumulative number of rotations, etc., of the image bearing member are obtained and used as life expectancy indices. With regard to the image bearing member installed in the target machine, the replacement timing of the image bearing member is set based not on individual image forming apparatuses but on the life expectancy indices.
However, exactly when the image bearing member has come to the end of its working life depends greatly on the environment and conditions of usage of individual image forming apparatuses. Therefore, if the replacement timing of the image bearing member is fixed, there is a risk that the image bearing member breaks down before the replacement timing. When the image bearing member has come to the end of its working life before its replacement timing, it is probable that defective images are output. In such a case, after replacing the image bearing members the defectively imaged item must be printed again.
It is possible to set the replacement timing of the image bearing member in any usage environment and condition early enough to avoid continuing printing with a defective image bearing member. However, as a result, a number of image bearing members are likely to be replaced prematurely, which is uneconomical and moreover wasteful.
Therefore, published Japanese patent application publication nos. 2009-92709 (JP-2009-92709-A) and JP-H05-100517-A describe determining whether an image bearing member has come to the end of its working life or predicting the life expectancy thereof based on readings of the image bearing member in use in each image forming apparatus. Thus, JP-2009-92709-A describes a device that detects the difference between the charging voltage of the first round and that of the second round after the start of charging of the image bearing member in rotation as a delay of charging that exceeds a predetermined allowance, and predicts when the image bearing member comes to the end of its working life based on the detection results.
JP-H05-100517-A describes a device that measures the surface voltage of a charged image bearing member twice at the same position, once before and once after a single rotation (VSO and VS1), without irradiation of the image bearing member with light or application of a developing bias or a transfer bias, to obtain two voltage readings of the charge (VSO and VS1), and predicts the life expectancy of the image bearing member from the difference between VSO and VS1. In this image forming apparatus, the calculated comparison “VSO−VS1” is defined as the dark decay amount VDD of the image bearing member at this point in time of the target image bearing member, and the life expectancy of the image bearing member is predicted from a relation between a dark decay VDDS of a new image bearing member and a preset, predetermined dark decay limit amount VDDLimit
In the image forming apparatus described in JP-2009-92709-A mentioned above, the life expectancy of the image bearing member is predicted based on detection of a charging delay, that is, the difference in the voltage at the surface of the image bearing member between the first rotation and the second rotation of the image bearing member after starting charging the image bearing member.
In theory, it is possible to be aware of the degree of charging delay at the detected portion on the surface of the image bearing member, i.e., how much the voltage at the surface of the image bearing member by the charging falls short of the target voltage for one rotation, from such a simple difference in the post-charging voltage between the first round and the second round. Therefore, according to the image forming apparatus described in JP-2009-92709-A mentioned above, the life expectancy of the image bearing member related to deterioration of the image quality caused by the charging delay can in theory be predicted.
However, in practice, the residual image is caused by the difference in the degree of the transfer impact on the image bearing member between the portion where no toner was attached and the portion where toner was attached. Therefore, it is not possible to be aware of the difference in the degree of the transfer impact from the simple difference in the post-charging voltage between the first round and the second round. Therefore, the image forming apparatus described in JP-2009-92709-A mentioned is not able to predict the life expectancy of the image bearing member ended by production of defective images with a residual image or determine whether the life of the image bearing member is over because of the production of such defective images.
The same is true in the case of the image forming apparatus described in JP-H05-100517-A mentioned above. That is, it is not possible to predict the life expectancy of the image bearing member ended by production of defective images with a residual image or determine whether the life of the image bearing member is over because of the production of such defective images.